Valley-fill circuits have long been recognized as a simple, low-cost option for providing power factor correction in power supplies and electronic ballasts that operate from a source of conventional AC power. The general idea of valley-fill power factor correction is to improve the shape of the current drawn from the AC line by increasing the conduction period of the rectifier diodes. This is achieved by maintaining a voltage on the bulk capacitors that is equal to a fraction of the peak value of the AC line voltage.
As illustrated in FIG. 1, a conventional valley-fill circuit requires a number of diodes (D.sub.1, D.sub.2, D.sub.3) and two bulk capacitors (C.sub.1, C.sub.2). For most applications, the bulk capacitors are high capacitance, high voltage electrolytic capacitors (e.g. 47 microfarads, 250 volts) and are therefore quite costly. Thus, significant impetus exists for alternative valley-fill approaches which require only a single bulk capacitor and a low number of peripheral components.
A modified valley-fill circuit that requires only a single bulk capacitor is described in FIG. 2. This approach uses energy from the resonant inductor (L.sub.R) to charge the bulk capacitor (C.sub.B). However, this approach is problematic since it requires a secondary winding on the resonant inductor, which adds significant cost and complexity to the resonant inductor and degrades the energy efficiency of the ballast.
Another valley-fill approach that requires only a single bulk capacitor is described in FIG. 3. This approach, which uses the positive half-cycles of the lamp current, I.sub.LAMP, to charge the bulk capacitor (C.sub.B), is less expensive and more efficient than the approach of FIG. 2. However, it has two serious disadvantages. First, as the voltage across the bulk capacitor (V.sub.BULK) is a fixed fraction of the peak value of the AC line voltage, this approach affords no option of designing for a bulk capacitor voltage that provides an optimal compromise between the competing requirements of power factor correction (which improves with lower values of V.sub.BULK) and lamp current crest factor (which is lower for higher values of V.sub.BULK). Secondly, this approach allows a substantial DC current to flow through the lamp for a considerable period of time after AC power is applied to the ballast. That is, following initial application of AC power to the ballast, a DC current will flow through the lamp during the period of time it takes for the bulk capacitor to initially charge up. The consequence of this DC current flowing through the fluorescent lamp for such a considerable period of time is migration of mercury from one end of the lamp to the other. Since this DC current will flow in the same direction each time the ballast is turned on, the migration effect is cumulative over time (unless the lamp is periodically removed and reinstalled in a reverse direction) and will significantly reduce the useful life of the lamp.
It is therefore apparent that an electronic ballast with a modified valley-fill circuit that requires only a single bulk capacitor, that allows for a design-adjustable value of the bulk capacitor voltage, that preserves lamp life by minimizing migration effects in the lamp, that is realizable with few components, and that is highly energy efficient, would represent a considerable advance over the prior art.